Picking a generator

Rngr275 · August 2012

I am struggling with the decision of choosing a generator for my soon to be installed Off-Grid system.

Here is the system:
The inverter/ battery system consists of:
1) 1- Outback VFX 3648 inverter
2) 1- Midnite 175 STS-L E-panel with circuit breakers for the inverter, charge controller and TM2020
3) 1- Outback Mate inverter programming module
4) 1- Set of 4/0 x 10' UL listed inverter battery cables
5) 1- Outback FNDC battery monitor
6) 16- Trojan L16-RE series 370 amp hour, 6 volt deep cycle batteries (740 amp hours @ 48VDC / 35,520 kWhrs AC)
7) 16- 2/0 x 12" battery interconnect cables
8) 1- 300 volt AC SPD surge arrestor
The PV system consists of:
1) 16- 255 watt PV modules (established name brand, to be determined by availability)
2) 2- DP&W 8-module, adjustable tilt pole top mounts (4-6 week lead time from factory)
3) 2- 6" x 21' schedule 80 pipes for array mounts (non-galvanized)
4) 1- Classic 200 charge controller
5) 1- Midnite MNPV12-250 combiner box (for 6 sub arrays)
6) 4- Midnite MNEPV15-300 combiner breakers, 300 volt rated
7) 1- 300 volt DC SPD surge arrestor

First I'm not sure how to determine the correct size (i.e watts/A output needed). Answering the first question could make the type/brand question easier. I am considering an Ecogen from Generac for a couple reasons: 1, Made for off grid (so they say) and I am joining a group where I can get propane for a reasonable price. But I will have to have a good size LP tank installed to feed the generator, HW heater, and stove.Price is ~$3500+ the $1500 install + LP tank. On the other hand I was looking at a 7Kw Genset (Powered by Honda) to both charge the system and move around the farm and to use in my small wood shop as needed. Obvious disadvantage (to me) gas is expensive and I will need to keep more on hand, louder than the Ecogen, but the price is ~1K. If I got the Ecogen I would still need to buy a portable sometime for the reasons stated above but I may choose a smaller sized. I currently have a Honda2000i but it can't run my Radial arm saw our my smallish compressor.

Any suggestions on 1)What size and 2) which way to go (LP,Gas)?

Thanks
McD

Cariboocoot · August 2012

Re: Picking a generator

Your generator needs to be able to supply two things: enough current to charge the batteries at maximum rate and enough power above that to handle the loads that will be running at the same time. The more loads you can control (turn off) the smaller gen you can use and the less fuel.

So you are looking at providing 74 Amps @ 48 Volts or 3552 Watts, plus the charger efficiency compensation (about 90%) or up to 4kW just to charge the batteries if they are seriously depleted. Your Honda 2000 isn't going to do this. Add some "load overhead" and you are looking at at least a 5kW genset.

Which fuel? Well which is cheapest for you? You ave to have some numbers on the fuel efficiency of the generator, which can be hard to come by. Then you can compare the kW hours it produces to the amount of fuel it uses and how much that fuel will cost and get a $ per kW hour figure. It may also be better to use a more expensive fuel if that is more convenient. LPG costs less but is less efficient, however it stores well and you'd already have a big supply on hand for the appliances. You don't have to pour it from can to tank with a permanent install. Not fun in NY Winters.

Hondas are extremely good quality equipment. I can't say the Generac stuff has all had the same sort of reviews.
There are numerous threads on the forum about generators and people's satisfaction with them. Unfortunately I'm on the "wrong" computer and haven't got my sixteen million bookmarks handy.

BB. · August 2012

Re: Picking a generator

Remember too that propane has less energy per gallon vs gasoline (and diesel is more energy per gallon vs gasoline). Around 84,000 to 93,000 BTU per gallon vs 111,000 to 114,000 BTU per gallon for gasoline. So, while LPG may cost less per gallon, a genset will use more fuel to make up for the lower energy content (although, if no road taxes, it LPG may still cost way less).

The Ecogen seems to have some folks here that like them. You can use Google to search the forum. Type "ecogen site:wind-sun.com" (without the "") (here is a google search link) and you should get a lot of hits on various threads.

It probably would not hurt to have a diversity of fuel and a backup genset if you have critical needs (ability to run the tools during if the first generator fails, ability to get backup fuel supply--i.e., can get gasoline but no propane delivery, etc.).

That said, propane is going to be the easiest to store (I keep 20 gallons of emergency gas, recycle to the car every year and refill with stabilizer). But watch the tank costs. Somebody said a few years ago that their propane supplier decided to charge a tank rental fee, then jacked the price up over the years until now people are considering dropping propane service.

Also, most gensets at 50% or less power will run about 50% fuel flow (Gallons per hour). So--I am a big fan of "not too big" of genset. Running a 7kW genset is going to use (probably) over a 1 GPH for propane at 3.5 kW or less loads. The Honda eu2000 will use ~0.25 GPH for a 1,500 watt load (down to 9+ hours per 1.1 gallons of gas to supply 400 watts average).

So, you may end up with two or more gensets to match the loads needing to be powered at the time for optimum fuel economy. I would suggest a Amp/Watt/kWH meter on your generator circuit(s) so you can monitor the power usage and even calculate the kWH/Gallon fuel usage.

-Bill

Cariboocoot · August 2012

Re: Picking a generator

I have not used one, but Honda does have some larger inverter-generators like this 4500 Watt unit: http://powerequipment.honda.ca/generators/inverter-series/em5000isc2/specs

Contrary to what some people may say, the efficiency of output is not the only consideration in a generator. In the case of the inverter types the over-all fuel consumption is less than a conventional generator even the the output per quantity of fuel consumed may not be as good. They shine when the loads reduce, which is an inevitable state when recharging batteries (you only need the maximum power at the start).

Since others have converted their smaller Hondas to propane it should be possible to do the same with the larger units and get the best of both worlds.

Vic · August 2012

Re: Picking a generator

For portable use, LP (Propane) powered gensets are a bit of a pain. So as noted by BB Bill, having several gensets of differing sizes and fuel sources is probably going to be very convenient. Would expect that one would find at least one gasoline-fueled genset very convenient. They do need to have fuel poured into their tanks, unless fueled from a nurse tank. But if you have gasoline fueled vehicles on the farm, it is nice to have one or more gasoline fueled gensets.

Honda brand gensets are very solid, and are very conservatively rated. (the Honda brand generators I've looked at have a Surge rating of 20 or on some, 30 minutes -- much better than any of the inexpensive generators). Have used Honda brand genssets here, of varying sizes, and all of them have been very reliable and trouble free. We do use an Inverter genset, similar to the one linked by Marc, installed as the backup genset to the main Diesel, and it is quiet and fuel stingy, but needs fuel poured into its tank.

Regarding the Generac Ecogen 6500, or 6.5 (forget), some here do use them. One member reported a motor siezure on one at abaout 41 hours, but was being replaced under warranty. Elsewhere, there have been reports of users having diffficulty doing the Field Upgrade from 120 VAC to 240 VAC output. The issue was incorrect output voltage after the upgrade. Something about a new Control board or similar being needed. It is quite possible this has been resolved by Generac by now.

Some extended discussion of the Generac Ecogen here:
http://www.outbackpower.com/forum/viewtopic.php?f=17&t=4591

There is no simple answer to the question of just which genset, or even fuel source for gensets. Many trade-offs. Good Luck, Vic

Rngr275 · August 2012

Re: Picking a generator

I understand that the brand and fuel choice is never a clear cut choice. I have to decide which one but it sounds like I need one in the 5-7Kw to get the amperage I need for the 740Ah battery bank.

Toby · August 2012

Re: Picking a generator

Can someone explain this exchange to me. I'm trying to understand a similar issue and now this has me confused.

Rngr275 wrote: 16- Trojan L16-RE series 370 amp hour, 6 volt deep cycle batteries (740 amp hours @ 48VDC / 35,520 kWhrs AC)

Cariboocoot replied: So you are looking at providing 74 Amps @ 48 Volts or 3552 Watts

vtmaps · August 2012

Re: Picking a generator

Toby wrote: »

Can someone explain this exchange to me. I'm trying to understand a similar issue and now this has me confused.

Rngr275 wrote: 16- Trojan L16-RE series 370 amp hour, 6 volt deep cycle batteries (740 amp hours @ 48VDC / 35,520 kWhrs AC)

Cariboocoot replied: So you are looking at providing 74 Amps @ 48 Volts or 3552 Watts

Recommended to charge batteries at 10% of their amphour rating. --vtMaps

CATraveler · August 2012

Re: Picking a generator

Toby wrote: »

Can someone explain this exchange to me. I'm trying to understand a similar issue and now this has me confused.

Rngr275 wrote: 16- Trojan L16-RE series 370 amp hour, 6 volt deep cycle batteries (740 amp hours @ 48VDC / 35,520 kWhrs AC)

Cariboocoot replied: So you are looking at providing 74 Amps @ 48 Volts or 3552 Watts

8 batteries in series gives 48V and then in parallel with the next 8 gives 740AH. At a 10% charge rate you would need 74A. Add effeciency and you'll need about a 4-5K gen and a 48V charger.

Toby · August 2012

Re: Picking a generator

Ok, I see it now. I could not figure out why the difference between 740 and 74 other than the missing zero.

Is 10% used for charging by solar panels too or only for electrically powered chargers?

BB. · August 2012

Re: Picking a generator

The general rule of thumb for Flooded Cell batteries (and does work well for AGM's too) is 5% to 13% rate of charge (based on 20 Hour back capacity).

5% or C/20 minimum--Below that, it takes a long time to recharge a bank (risk of sulfation). Self discharge and other losses become "significant" loads themselves. Roughly, 5% is required for equalization. (C/20 is typically the maximum rate of charge for GEL type batteries--Which are generally not recommended for solar/off grid use).
10% or C/10 nominal--Good sized charger/array. Quickly recharges batteries. For for most climates, provides more power than needed on most days with a 2 day/50% maximum discharge battery bank design (except during darkest winter).
13% or C/8--The maximum rate of charge recommended for deeply cycled batteries (such as forklift duty). Batteries can overheat above this rate of charge. Also, more or less, the maximum rate suggested for solar panels/charging... Batteries will be fully charged by the middle or afternoon for most people--waste of power beyond that (unless you have heavy daytime loads such as irrigation pumping, etc.).
25% or C/4--Probably the maximum you would want for an AC battery charger. Watch battery charger and use a remote battery temperature sensor on the charger (if it has one).

Note that AC battery chargers frequently need all sorts of fudge factors when running off a genset.

Typical battery chargers are about 80% efficient and may have as bad as 67% power factor. So--When computing the size of generator--It may look something like this:

100 Amp charging current * 59 volt battery voltage charging * 1/0.80 charger eff * 1/0.67 PF = 11,007 VA rated genset
100 amps * 59 volts = 5,900 watts into battery bank

Note, the 0.67 and 0.80 numbers--Many of the Inverter/Chargers have chargign efficiency close to 90% and PF near 1.0--Stand alone chargers with those numbers are more difficult to find.

Here is a very nice thread that goes through the selection process for a smaller AC battery charger running off a 1,600 watt genset and why this all matters:

Question about battery charger selection with EU2000 generator.

-Bill

vtmaps · August 2012

Re: Picking a generator

Toby wrote: »

Is 10% used for charging by solar panels too or only for electrically powered chargers?

General rule of thumb: charge rate should be 5% - 13% of battery capacity. Too high and you may overheat the battery. Too low and you may run out of fuel (generator) or daylight (panels) before the battery is charged. Some batteries (AGM) can take higher current, others (gell) require lower current.

If you have other loads (besides charging the battery) you may want to supersize your generator or panels accordingly.

--vtMaps

BB. · August 2012

Re: Picking a generator

As vtMaps says--Over sizing the generator/AC battery charger will cost you fuel. A small genest+charger will be running the generator at ~50% rated load for most of the charging cycle.

A huge generator and AC battery charger, will quickly move from bulk to absorb--where the current tapers from maximum to near 1-2% of battery AH (end of charge). The more time the genset spends below ~50% of rated output, the more fuel is "wasted".

Also, the PF number and VA... The generator needs to be sized for Volt*Amps (VA or kVA). But the fuel burned will be based on Watts (power=VA*PF). So, "bad" PF is also a burden on fuel usage too (as is poor charger efficiency).

-Bill

vtmaps · August 2012

Re: Picking a generator

BB. wrote: »

As vtMaps says--Over sizing the generator/AC battery charger will cost you fuel.

Actually, that's not what I said, but perhaps I should have because it is certainly true. I said that if the charge rate is too low you may run out of fuel before the battery is charged. Generator size needs to be part of a balanced system design.

BB. wrote: »

Also, the PF number and VA... The generator needs to be sized for Volt*Amps (VA or kVA). But the fuel burned will be based on Watts (power=VA*PF). So, "bad" PF is also a burden on fuel usage too

I don't follow this... the PF would have to be incredibly low (below 0.5) before you were forced to use a generator at less than 50% of its capacity.
--vtMaps

BB. · August 2012

Re: Picking a generator

Battery chargers are variable loads--So, as the battery hits "bulk" (charger in "constant or current limited mode) (typically around 80-90% state of charge), then the current starts to taper down (as the charger is now in "constant" voltage mode.

So, the poor PF still increases the current drawn from the generator (1/0.67 = 1.49x more current, but no more power, and no "extra fuel" needed) that cannot be used for other things (say running the shop, pumping, etc.). So, you end up with a 1.49x larger genset than you need, that, on average, runs more time at less than 50% rated load where "fuel economy" is poor (very roughly, a gasoline/propane genset will use around 50% of rated fuel flow (gallons per hour) for 50% to zero% of rated output).

It just adds to the "losses and fudge factors" that cause grief to people running off grid power systems.

Regarding AC vs DC rated switches--Of course, you are right vtMaps. However, I do not think the typical DC Ground Fault systems used today are safel, but are quite dangerous instead--And that is "required by code".

-Bill

Vic · August 2012

Re: Picking a generator

McD,

Here is an additional Thread on the OB Forum regarding Roy Salisbury's experience with Ecogen reliability, or lack thereof:
http://www.outbackpower.com/forum/viewtopic.php?f=17&t=6304

And regarding the effects of Power Factor on gensets during battery charging; My opinion is that that the genset needs to be able to deliver the peak currents, often over a period of hours. So there are the effects of the peak current on the alternator, regulator and protective systems, the genset must be capable of providing these peaks over an extended time. Poor PF often distorts the AC waveform, which can result in even less power being delivered to the battery charger/battery bank.

Having one stout genset available for charging the battery bank, especially when the SOC is on the low side is must, if one values one's battery bank. IMHO, this is where much of the recommendation for higher range of recharge currents for batteries comes from -- recharge reasonably quickly when SOC is low-ish. These high recharge currents are not generally needed or even used in daily recharge when batteries are lightly cycled. This capability should be available, if needed.

As a note on the EQ currents required on 7-year old banks here; at the beginning of an EQ, have not seen more than 2.5% of capacity required. This is with bank temps in the 25-28 degree C range, and EQ voltages on the lower end of spec. Know that as the bank ages, and when an EQ is really needed, required currents WILL increase ... again very good idea to have more recharge/EQ power avail when needed. Opinions Vic

Toby · August 2012

Re: Picking a generator

That's a lot information to digest. I do appreciate it.

Let me see if I'm on track. Looking at two different inverter/chargers, one has a maximun continious charger output of 60adc, and the other 70adc. So, working the math the other way, the first one could charge 600ah battery and the other 700ah.

BB. · August 2012

Re: Picking a generator

Correct if you choose the 10% rate of charge.

-Bill

Toby · August 2012

Re: Picking a generator

Ok, glad I got that correct. One more.

The Magnum MS4448PAE has a continious charger output of 60adc. The AC current input at the rated DC charger output is 17.5 aac per leg at 120/240 vac. The 35 aac, for both legs of 240vac, would require at least an 8400 watt generator (not including any surge)?

vtmaps · August 2012

Re: Picking a generator

Toby wrote: »

The AC current input at the rated DC charger output is 17.5 aac per leg at 120/240 vac. The 35 aac, for both legs of 240vac, would require at least an 8400 watt generator (not including any surge)?

17.5 amps at 120 volts = 2100 VA. For both legs that is 2100 x 2 = 4200 VA.
The magnum charger is PF corrected so 4200 VA = 4200 watts.

Consider the output: 60 amps DC x ~60 volts (to the 48 volt) battery = ~3600 watts.

--vtMaps

Toby · August 2012

Re: Picking a generator

Thanks vtmaps. I misunderstood the amps per leg.

Picking a generator

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